Cochlear function changes throughout the human lifespan in a continuum of maturation and aging. Otoacoustic emissions (OAE) provide a non-invasive window into the cochlea and the mechanisms driving these changes. Here, we apply OAEs to map age-related shifts in cochlear mechanics and explore the potential for new probes of hearing. In Aim 1 we exploit the robust reflection generated by neonatal ears and record stimulus frequency (SF) OAEs, a reflection-source emission, in newborns. SFOAEs have been linked to cochlear tuning and gain yet have never been characterized in newborns, mostly because of their need for prohibitively long protocols to achieve adequate signal-to-noise ratios. This problem is rendered tractable by novel algorithms allowing for the rapid presentation of stimuli as sweeping tones. We will characterize the SFOAE for the first time in a large cohort of newborn ears and adapt the innovative swept-tone algorithm for optimal use within this age group. Aim 2 studies the apical portion of the newborn and adult cochlea, a region representing nearly half of our audible frequency range, yet poorly understood and wholly unexplored during maturation. In newborn ears, non-adult-like DPOAE phase has been observed for low-frequency signals; we hypothesize that subtle immaturities in the apical frequency-place map may account for these findings. Here, seek converging evidence from reflection- and distortion-source OAEs, record and model DPOAE apical phase maps, and conduct histological analysis of human temporal bones to link apical anatomy to OAE features during development. Changes in cochlear mechanics do not cease after infancy; the aging ear experiences documented changes in the health of its sensory cells and in its metabolic environment, producing altered cochlear efficiency. Cochlear-based deficits contribute to perceptual difficulties common to older listeners. In Aim 3, we hypothesize that one such deficit is the loss of cochlear nonlinearity in the aging ear, and we define how OAE-based metrics of nonlinearity change from middle-age through senescence. Furthermore, we hypothesize that the cochlea becomes more rough with aging, possibly accounting for its ability to generate reflection emissions despite declining cochlear health. These three aims apply diverse experimental and theoretical tools and break new ground; they characterize SFOAEs in newborns as a first step in exploring their potential as a neonatal hearing screen, probe the oft-neglected apical half of the human cochlea where neonatal immaturities have been observed, and define how cochlear nonlinearities and OAE generation change in the aging ear.